Screen printing is widely used in the photovoltaic industry to form metal contacts of solar cells mainly due to its low cost and high throughput. Despite the disadvantages in high contact resistance, high bulk resistivity, low line aspect ratio, and limitation on cell design, silver (Ag) and AlAg is the main screen printing metal used today due to its low resistivity and availability. Although copper (Cu) has low resistivity and is much more abundant and lower cost, it is not chosen because its difficulty in screen printing.
Conventional photovoltaic cells are formed from monocrystalline, polycrystalline, or multicrystalline silicon (Si). Monocrystalline, multicrystalline, and polycrystalline materials are entirely or almost entirely crystalline, with no or almost no amorphous matrix. Photovoltaic cells fabricated from substantially crystalline material are conventionally formed of wafers sliced from a Si ingot, or Si ribbons pulled from Si melt.
Light-induced (illumination) plating methods onto solar cells were introduced in the 1970s.
U.S. Pat. No. 4,144,139 describes the light-induced plating method for solar cells under illumination. Plating electrical contacts onto one or more surfaces of a solar cell having an electrical junction therein is accomplished by immersing the cell in an electrolyte and exposing it to light so that platable ions in the electrolyte will be attracted to an oppositely charged surface of the cell. This method uses no external power supply and need no electrical contact to deposit silver on the front side n-emitter while dissolving silver from the back side surface. The deposition is carried out within one tank setup.
U.S. Pat. No. 4,251,327 describes a method for electroplating a metallic layer onto the surface of a photovoltaic device absent any external electrical contacts to the surface. The photovoltaic device is placed in an electrolytic plating bath where it is illuminated with electromagnetic radiation to which the device is photovoltaically responsive. Plating from the electrolytic bath results from current flow generated in the device itself.
This patent describes an improved embodiment over U.S. Pat. No. 4,144,139 by using immersion palladium deposition followed by 300° C. silicidation process, followed by electroless palladium deposition and further silicidation, and then followed by silver plating. It also describes a second embodiment using external current supply to the backside of the wafer to have both sides plated. The deposition is also carried out within one tank.
Dube, C. E. and Gonsiorawski, R. C., in their publication “Improved contact metallization for high efficiency EFG polycrystalline silicon solar cells” at the Twenty First IEEE Photovoltaic Specialists Conference, describe plating copper (Cu) with dissolving Cu on the backside of the solar cell by using laser patterning to create dense contact vias on the front.
The advantages of the light-induced plating method compared with electroless deposition and the above-mentioned conventional electroplating techniques are listed in the following Table.
TABLEElectrodepositionElectrodepositionElectroless depositionunder IlluminationRequires electricalNo electricalNo electricalcontact to eachcontact neededcontact neededpattern for depositionExternal powerNo need for external powerNo need for externalsupply for carryingsupply, but need thermalpower supply, norout depositionheating of the solutionthermal heating.Potentially canuse sunlightLow cost & highHigh cost & low throughputLow cost & highthroughputdue to the need for complexthroughputbath monitoringPure materialsMore resistivePure materialsdepositedmaterials due to inclusiondepositedof impurities such asPhosphorus & boron
A disadvantage of the basic methods described in U.S. Pat. No. 4,144,139 and U.S. Pat. No. 4,251,327 is that the deposition of metal and dissolution of metal occurs in the same solution. This method intrinsically makes process monitoring very difficult, since it does not provide the control on the current/charge during the process, which makes it difficult to monitor the amount of deposit. Furthermore, the chemical components used in the plating solution must be compatible with both the plating process and the dissolution process on the backside of the substrate.
Another disadvantage of the single tank approach is that it is difficult to deposit metals that will not dissolve in their own plating solution. When the metal being deposited does not dissolve in its own plating solution, oxygen evolution will occur instead of metal dissolution on the back. The potential driving force needed for the deposition of a metal coupled with oxygen evolution is much larger than the potential driving force needed for the deposition of a metal coupled with the dissolution of a metal. Nickel and palladium are examples of these metals.
In order to deposit nickel or palladium by a single-tank setup, an external voltage must be supplied to carry out the deposition.
Thus, there is a need for an efficient low cost and high through-put process, one in which two separate tanks are used during the process, and in which the current/charge passed between the two separate tanks can be monitored and the structure/surface is protected from corrosion, and there is no need for external voltage supply for the electrodeposition other than light illumination.